Cathode for Lithium Secondary Battery and Lithium Secondary Battery Including the Same

ABSTRACT

A cathode for a lithium secondary battery includes a cathode current collector, and a cathode active material layer including a first cathode active material layer and a second cathode active material layer sequentially stacked on the cathode current collector. The first cathode active material layer includes first cathode active material particles having a secondary particle structure, and the second cathode active material layer includes second cathode active material particles having a single particle shape. A ratio of a SPAN value of the second cathode active material particles relative to a SPAN value of the first cathode active material particles is in a range from 2 to 4.5.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application No.10-2021-0020435 filed Feb. 16, 2021, the disclosure of which is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a cathode for a lithium secondarybattery and a lithium secondary battery including the same. Moreparticularly, the present invention relates to a cathode for a lithiumsecondary battery including a lithium metal oxide-based cathode activematerial, and a lithium secondary battery including the same.

2. Description of the Related Art

A secondary battery which can be charged and discharged repeatedly hasbeen widely employed as a power source of a mobile electronic devicesuch as a camcorder, a mobile phone, a laptop computer, etc., accordingto developments of information and display technologies. Recently, abattery pack including the secondary battery is being developed andapplied as a power source of an eco-friendly vehicle such as a hybridautomobile.

The secondary battery includes, e.g., a lithium secondary battery, anickel-cadmium battery, a nickel-hydrogen battery, etc. The lithiumsecondary battery is highlighted due to high operational voltage andenergy density per unit weight, a high charging rate, a compactdimension, etc.

For example, the lithium secondary battery may include an electrodeassembly including a cathode, an anode and a separation layer(separator), and an electrolyte immersing the electrode assembly. Thelithium secondary battery may further include an outer case having,e.g., a pouch shape.

A lithium metal oxide may be used as a cathode active material of thelithium secondary battery preferably having high capacity, power andlife-span. However, as an application range of the lithium secondarybattery has been expanded, stability in a harsh environment such as hightemperature or low temperature environment is further required. Forexample, when the lithium secondary battery or the cathode activematerial is designed for high capacity, cell stability at hightemperature may be deteriorated, and life-span properties of the batterymay also be deteriorated. For example, Korean Publication of PatentApplication No. 10-2017-0093085 discloses a cathode active materialincluding a transition metal compound and an ion adsorbing binder, whichmay not provide sufficient life-span and stability.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided acathode for a lithium secondary battery having improved operationalstability and reliability.

According to exemplary embodiments, there is provided a lithiumsecondary battery including a cathode having improved operationalstability and reliability.

According to exemplary embodiments, a cathode for a lithium secondarybattery includes a cathode current collector, and a cathode activematerial layer including a first cathode active material layer and asecond cathode active material layer sequentially stacked on the cathodecurrent collector. The first cathode active material layer includesfirst cathode active material particles having a secondary particlestructure, and the second cathode active material layer includes secondcathode active material particles having a single particle shape. Aratio of a SPAN value defined as Equation 1 of the second cathode activematerial particles relative to a SPAN value defined as Equation 1 of thefirst cathode active material particles is in a range from 2 to 4.5.

SPAN=(D90−D10)/D50  [Equation 1]

In Equation 1, D10, D50 and D90 are particle diameters at cumulativevolume percents of 10%, 50% and 90%, respectively, in a cumulativevolume particle diameter distribution.

In some embodiments, the ratio of the SPAN value of the second cathodeactive material particles relative to the SPAN value of the firstcathode active material particles may be in a range from 2 to 3.5

In some embodiments, the SPAN value of the second cathode activematerial particles may be in a range from 0.7 to 2, and the SPAN valueof the first cathode active material particles may be less than 1.

In some embodiments, the SPAN value of the second cathode activematerial particles may be in a range from 0.9 to 1.7, and the SPAN valueof the first cathode active material particles may be in a range from0.3 to 0.6.

In some embodiments, each of the first cathode active material particlesand the second cathode active material particles may include a lithiummetal oxide containing nickel, and a content of nickel included in thefirst cathode active material particles may be greater than or equal toa content of nickel included in the second cathode active materialparticles.

In some embodiments, each of the first cathode active material particlesand the second cathode active material particles may further containcobalt and manganese, a molar ratio of nickel among nickel, cobalt andmanganese in the first cathode active material particles may be 0.6 ormore, and a molar ratio of nickel among nickel, cobalt and manganese inthe second cathode active material particles may be 0.5 or more.

In some embodiments, the molar ratio of nickel among nickel, cobalt andmanganese in the first cathode active material particles may be 0.8 ormore, and the molar ratio of nickel among nickel, cobalt and manganesein the second cathode active material particles may be 0.6 or more.

In some embodiments, the first cathode active material particles mayinclude a concentration non-uniformity region of at least one element ofnickel, cobalt and manganese between a central portion and a surfaceportion.

In some embodiments, each of nickel, cobalt and manganese contained inthe second cathode active material particles may not form aconcentration gradient.

In some embodiments, an average particle diameter of the second cathodeactive material particles may be smaller than an average particlediameter of the first cathode active material particles.

In some embodiments, a thickness of the second cathode active materiallayer may be smaller than a thickness of the first cathode activematerial layer.

According to exemplary embodiments, a lithium secondary battery includesa case, and an electrode assembly accommodated in the case. Theelectrode assembly includes an anode and the cathode for a lithiumsecondary battery according to embodiments as described above facing theanode.

A lithium secondary battery according to exemplary embodiments mayinclude a cathode active material layer having a multi-layeredstructure. The cathode active material layer may include a first cathodeactive material layer including a cathode active material particlehaving a secondary particle structure, and a second cathode activematerial layer including a cathode active material particle having asingle particle shape.

The first cathode active material layer may be disposed to be adjacentto a current collector to realize high-power and high-capacityproperties, and the second cathode active material layer may be disposedat an outer portion of the cathode to improve thermal stability andpenetration stability.

In exemplary embodiments, a ratio of a SPAN value of the first cathodeactive material particles included in the first cathode active materiallayer and a SPAN value of the second cathode active material particlesincluded in the second cathode active material layer may be adjusted, sothat particle cracks generated during an electrode pressing process maybe effectively prevented. Accordingly, gas generation due to a sidereaction caused when high-temperature charging/discharging may berepeated may be suppressed to remarkably improve life-span properties ofthe secondary battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating a cathode for alithium secondary battery in accordance with exemplary embodiments.

FIGS. 2 and 3 are a schematic top planar view and a schematiccross-sectional view illustrating a lithium secondary battery inaccordance with exemplary embodiments.

FIGS. 4 and 5 are SEM images showing a cross-section and a surface of afirst cathode active material particle, respectively, prepared accordingto Example 1.

FIGS. 6 and 7 are SEM images showing a cross-section and a surface of asecond cathode active material particle, respectively, preparedaccording to Example 1.

DESCRIPTION OF THE INVENTION

According to exemplary embodiments of the present invention, a cathodefor a secondary battery including a multi-layered structure of a firstcathode active material layer and a second cathode active material layerthat may include different cathode active material particles isprovided. Further, a lithium secondary battery including the cathode andhaving improved power and stability is also provided.

Hereinafter, the present invention will be described in detail withreference to the accompanying drawings. However, those skilled in theart will appreciate that such embodiments described with reference tothe accompanying drawings are provided to further understand the spiritof the present invention and do not limit subject matters to beprotected as disclosed in the detailed description and appended claims.

The terms “first” and “second” are used herein not to limit the numberor the order of elements or objects, but to relatively designatedifferent elements.

FIG. 1 is a schematic cross-sectional view illustrating a cathode for alithium secondary battery in accordance with exemplary embodiments.

Referring to FIG. 1, a cathode 100 may include a cathode active materiallayer 110 formed on at least one surface of a cathode current collector105. The cathode active material layer 110 may be formed on bothsurfaces (e.g., upper and lower surfaces) of the cathode currentcollector 105.

The cathode current collector 105 may include, e.g., stainless steel,nickel, aluminum, titanium, copper or an alloy thereof, and maypreferably include aluminum or an aluminum alloy.

In exemplary embodiments, the cathode active material layer 110 mayinclude a first cathode active material layer 112 and a second cathodeactive material layer 114. Accordingly, the cathode active materiallayer 110 may have a multi-layered structure (e.g., a double-layeredstructure) in which a plurality of cathode active material layers arestacked.

The first cathode active material layer 112 may be formed on a surfaceof the cathode current collector 105. For example, the first cathodeactive material layer 112 may be formed on each of the upper and lowersurfaces of the cathode current collector 105. As illustrated in FIG. 1,the first cathode active material layer 112 may directly contact thesurface of the cathode current collector.

The first cathode active material layer 112 may include a first cathodeactive material particle. The first cathode active material particle mayinclude a lithium metal oxide containing nickel and another transitionmetal. In exemplary embodiments, in the first cathode active materialparticle, nickel may be included in the highest content (molar ratio)among metals other than lithium, and the content of nickel among themetals other than lithium may be about 60 mol % or more.

In some embodiments, the first cathode active material particle may berepresented by the following Chemical Formula 1.

Li_(x)Ni_(a)M1_(b)M2_(c)O_(y)  [Chemical Formula 1]

In Chemical Formula 1, M1 and M2 may each include at least one of Co,Mn, Na, Mg, Ca, Ti, V, Cr, Cu, Zn, Ge, Sr, Ag, Ba, Zr, Nb, Mo, Al, Ga,W, Y or B, and 0<x≤1.2, 2≤y≤2.02, 0.6≤a≤0.99, 0.01≤b+c≤0.4.

In some embodiments, the first cathode active material particle mayfurther include at least one of cobalt (Co) and manganese (Mn). In anembodiment, M1 and M2 in Chemical Formula 1 may include cobalt (Co) andmanganese (Mn), respectively.

For example, nickel may serve as a metal associated with power and/orcapacity of a lithium secondary battery. As described above, the lithiummetal oxide having a nickel content of 0.6 or more may be employed asthe first cathode active material particle, and the first cathode activematerial layer 112 may be formed to be in contact with the cathodecurrent collector 105, so that high power and high capacity may beefficiently implemented from the cathode 100.

For example, manganese (Mn) may serve as a metal related to mechanicaland electrical stability of the lithium secondary battery. For example,cobalt (Co) may serve as a metal related to conductivity, resistance orpower of the lithium secondary battery.

In a preferable embodiment, 0.7≤a≤0.99 and 0.01≤b+c≤0.3 in considerationof achieving high power/capacity from the first cathode active materiallayer 112.

In an embodiment, in the first cathode active material particle, thecontent (molar ratio) of nickel may be about 0.8 or more. For example,the concentration ratio (molar ratio) of nickel:cobalt:manganese may beadjusted as about 8:1:1. In this case, conductivity and life-spanproperties may be supplemented by including cobalt and manganese insubstantially equal amounts while increasing capacity and power throughnickel of about 0.8 molar ratio.

In some embodiments, the first cathode active material particle mayinclude a concentration non-uniformity region. For example, the metalelement other than lithium included in the first cathode active materialparticle may have a local concentration non-uniformity or a localconcentration difference. For example, the metal element may include aconcentration non-uniformity region or a concentration gradient region.For example, the first cathode active material particle may include thelithium metal oxide in which a concentration gradient region or aconcentration non-uniformity region of at least one metal is formed.

In some embodiments, the first cathode active material particle mayinclude a concentration non-uniformity region between a central portionand a surface portion. For example, the first cathode active materialparticle may include a core region and a shell region, and theconcentration non-uniformity region may be included between the coreregion and the shell region. The concentration may be constant or fixedin the core region and the shell region.

In an embodiment, the concentration non-uniformity region may be formedat the central portion. In an embodiment, the concentrationnon-uniformity region may be formed at the surface portion.

In some embodiments, the first cathode active material particle mayinclude the lithium metal oxide having a continuous concentrationgradient from a center of the particle to a surface of the particle. Forexample, the first cathode active material particle may have a fullconcentration gradient (FCG) structure having a substantially entireconcentration gradient throughout the particle.

The term “continuous concentration gradient” used herein may indicate aconcentration profile which may be changed with a certain trend ortendency between the center and the surface. The certain trend mayinclude an increasing trend or a decreasing trend.

The term “central portion” used herein may include a central point ofthe active material particle and may also include a region within apredetermined radius from the central point. For example, “centralportion” may encompass a region within a radius of about 0.1 μm, 0.2 μm,0.3 μm or 0.4 μm from the central point of the active material particle.

The term “surface portion” used herein may include an outermost surfaceof the active material particle, and may also include a predeterminedthickness from the outermost surface. For example, “surface portion” mayinclude a region within a thickness of about 0.1 μm, 0.05 μm or 0.01 μmfrom the outermost surface of the active material particle.

In some embodiments, the concentration gradient may include a linearconcentration profile or a curved concentration profile. In the curvedconcentration profile, the concentration may change in a uniform trendwithout any inflection point.

In an embodiment, at least one metal except for lithium included in thefirst cathode active material particle may have an increasing continuousconcentration gradient, and at least one metal except for lithiumincluded in the first cathode active material particle may have adecreasing continuous concentration gradient.

In an embodiment, at least one metal included in the first cathodeactive material particle except for lithium may have a substantiallyconstant concentration from the central portion to the surface.

When the first cathode active material particle includes theconcentration non-uniformity region, the concentration (or the molarratio) of Ni may be continuously decreased from the central portion tothe surface or in the concentration non-uniformity region. For example,the concentration of Ni may be decreased in a direction from the centralportion to the surface within a range between about 0.99 and about 0.6.In an embodiment, the concentration of Ni may be decreased in thedirection from the central portion to the surface within a range betweenabout 0.99 and about 0.7, e.g., between 0.9 and 0.7.

In a case that M1 and M2 in Chemical Formula 1 are cobalt and manganese,respectively, a concentration of manganese may increase from the centralportion to the surface, or in the concentration non-uniformity region.The content of manganese may be increased in a direction to the surfaceof the particle, so that defects such as ignition and short circuit dueto a penetration occurring through the surface of the first cathodeactive material particle may be suppressed or reduced to increaselife-span of the lithium secondary battery.

In an embodiment, a concentration of cobalt may be fixed or constantthroughout an substantially entire region of the first cathode activematerial particle. Thus, improved conductivity and low resistance may beachieved while constantly maintaining a flow of current and chargethrough the first cathode active material particle.

In an embodiment, when M1 and M2 in Chemical Formula 1 are cobalt andmanganese, respectively, the concentration of cobalt may increase in thedirection to the surface in the concentration non-uniformity region. Inthis case, the concentration of manganese may be fixed or constantthroughout substantially the entire region of the first cathode activematerial particle. Thus, chemical and thermal stability may bemaintained throughout the entire region of the first cathode activematerial particle. For example, the concentration of cobalt may beincreased at the surface of the particle to improve power andconductivity at the surface portion.

For example, when the molar ratio of nickel:cobalt:manganese in thefirst cathode active material particle is adjusted to about 8:1:1 andthe first cathode active material particle includes the concentrationgradient, an overall average concentration in the particle may be about8:1:1.

In exemplary embodiments, the first cathode active material particle mayhave a structure or a shape of a secondary particle in which a pluralityof primary particles (e.g., 20 or more, 30 or more, 50 or more, or 100or more, etc.) are aggregated or assembled.

In this case, the first cathode active material particle may be formedthrough a co-precipitation method of a metal precursor. For example,metal precursor solutions having different concentrations may beprepared. The metal precursor solutions may include precursors of metalsto be included in the cathode active material. For example, the metalprecursors may be a halide, hydroxide or an acid salt of the metals.

For example, the metal precursors may include a lithium precursor (e.g.,lithium hydroxide, lithium oxide), a nickel precursor, a manganeseprecursor and a cobalt precursor.

In exemplary embodiments, a first precursor solution having a targetcomposition at the central portion (e.g., concentrations of nickel,manganese and cobalt at the central portion) of the first cathode activematerial particle and a second precursor solution having a targetcomposition at the surface (e.g., concentrations of nickel, manganeseand cobalt at the surface) of the first cathode active material particlemay each be prepared.

Thereafter, a precipitate may be formed while mixing the first andsecond precursor solutions. During the mixing, a mixing ratio may becontinuously changed so that a concentration gradient is continuouslyformed from the target composition at the central portion to the targetcomposition at the surface. Accordingly, primary particles of varyingconcentrations may be precipitated and aggregated to generate secondaryparticles having a concentration gradient within the particle.Accordingly, the structure of the first cathode active material particleincluding the above-described concentration gradient may be easilyobtained.

In some embodiments, the precipitate may be formed by adding a chelatingagent and a basic agent during the mixing. In some embodiments, theprecipitated may be heat-treated, and then may be mixed with the lithiumprecursor and heat-treated again.

In some embodiments, the first cathode active material particle may beprepared by a solid phase mixing/reaction, and a method of preparing thefirst cathode active material particle is not be limited to thesolution-based process as described above.

The first cathode active material particle may be mixed and stirredtogether with a binder, a conductive material and/or a dispersing agentin a solvent to form a first slurry. The first slurry may be coated onthe cathode current collector 105, and dried and pressed to obtain thefirst cathode active material layer 112.

The binder may include an organic based binder such as a polyvinylidenefluoride-hexafluoropropylene copolymer (PVDF-co-HFP),polyvinylidenefluoride (PVDF), polyacrylonitrile,polymethylmethacrylate, etc., or an aqueous based binder such asstyrene-butadiene rubber (SBR) that may be used with a thickener such ascarboxymethyl cellulose (CMC).

For example, a PVDF-based binder may be used as a cathode binder. Inthis case, an amount of the binder for forming the first cathode activematerial layer 112 may be reduced, and an amount of the first cathodeactive material particles may be relatively increased. Thus, capacityand power of the lithium secondary battery may be further improved.

The conductive material may be added to facilitate electron mobilitybetween the active material particles. For example, the conductivematerial may include a carbon-based material such as graphite, carbonblack, graphene, carbon nanotube, etc., and/or a metal-based materialsuch as tin, tin oxide, titanium oxide, a perovskite material such asLaSrCoO₃ or LaSrMnO₃, etc.

The second cathode active material layer 114 may be formed on the firstcathode active material layer 112. As illustrated in FIG. 1, the secondcathode active material layer 114 may be directly formed on an uppersurface of the first cathode active material layer 112, and may serve asa coating layer of the cathode 100.

The second cathode active material layer 114 may include second cathodeactive material particles. The second cathode active material particlemay include a lithium metal oxide containing nickel, cobalt and anothertransition metal.

In exemplary embodiments, each of the second cathode active materialparticles may have a single particle shape.

The term “single particle shape” herein may be used to exclude asecondary particle in which a plurality of primary particles areagglomerated. For example, the second cathode active material particlemay substantially consist of particles having the single particle shape,and the second particle structure having primary particles agglomeratedor assembled therein may be excluded.

The term “single particle shape” herein may not exclude a monolithicshape in which several (e.g., less than 20 or less than 10) independentparticles are adjacent or attached to each other.

In some embodiments, the second cathode active material particle mayinclude a structure in which a plurality of primary particles areintegrally merged to be converted into a substantially single particle.

For example, the second cathode active material particle may have agranular single particle shape or a spherical single particle shape.

In exemplary embodiments, the second cathode active material particlemay have a substantially constant or fixed concentration throughout anentire region of the particle. For example, concentrations of metalsexcept for lithium may be substantially fixed or constant from a centralportion of the particle to a surface of the particle in the secondcathode active material particle.

In some embodiments, the second cathode active material particle mayinclude nickel (Ni), cobalt (Co) and manganese (Mn). As described above,concentrations or molar ratios of Ni, Co and Mn may be substantiallyconstant or fixed throughout the entire region of the second cathodeactive material particle. For example, Ni, Co and Mn may not form aconcentration gradient in the second cathode active material particle.

In some embodiments, a concentration of nickel in the second cathodeactive material particle may be equal to or less than a concentration ofnickel in the first cathode active material particle. In a preferableembodiment, the concentration of nickel in the second cathode activematerial particle may be less than a concentration of nickel in thefirst cathode active material particle.

In an embodiment, the concentration of nickel in the second cathodeactive material particle may be fixed to be equal to or less than theconcentration of nickel at the surface of the first cathode activematerial particle, preferably less than the concentration of nickel atthe surface of the first cathode active material particle.

In some embodiments, the concentration of nickel in the second cathodeactive material particle may be equal to or more than the concentrationof nickel in the first cathode active material particle.

In some embodiments, a molar ratio of Ni among metals except for lithiumin the second cathode active material particle may be in a range fromabout 30% to about 99%. Within this range, sufficient thermal andpenetration stability may be obtained from the second cathode activematerial layer 114 without degrading capacity/power of the cathode 100.

In a preferable embodiment, the molar ratio of Ni among metals exceptfor lithium in the second cathode active material particle may be 50% ormore, e.g., in a range from 50% to 99%.

In an embodiment, the molar ratio of Ni among metals except for lithiumin the second cathode active material particle may be 60% or more, 70%or more or 80% or more.

In an embodiment, the molar ratios of Ni among metals except for lithiumin the first cathode active material particle and the second cathodeactive material particle may each be 80% or more.

In some embodiments, the second cathode active material particle mayinclude a lithium metal oxide represented by the following ChemicalFormula 2.

Li_(x)Ni_(a)Co_(b)Mn_(c)M4_(d)M5_(e)O_(y)  [Chemical Formula 2]

In the Chemical Formula 2 above, M4 may include at least one elementselected from Ti, Zr, Al, Mg, Si, B, Na, V, Cu, Zn, Ge, Ag, Ba, Nb, Gaor Cr. M5 may include at least one element selected from Sr, Y, W or Mo.In Chemical Formula 2, 0.9<x<1.3, 2≤y≤2.02, 0.313≤a≤0.99, 0.045≤b≤0.353,0.045≤c≤0.353 and 0.98≤a+b+c+d+e≤1.03.

As represented by Chemical Formula 2, in consideration of both capacityand stability of the lithium secondary battery, an amount of Ni may bethe largest of those of the metals except for lithium in the secondcathode active material particle, and the molar ratio of Ni in thesecond cathode active material particle may be less than that in thefirst cathode active material particle

In some embodiments, the second cathode active material particle may beprepared by a solid-state thermal treatment of the metal precursors. Forexample, the lithium precursor, the nickel precursor, the manganeseprecursor and the cobalt precursor may be mixed according to thecomposition of the Chemical Formula 2 above to form a precursor powder.

The precursor powder may be thermally treated in a furnace at, e.g., atemperature from about 700° C. to about 1200° C., and the precursors maybe merged or fused into a substantially single particle shape to obtainthe second cathode active material particle having the single particleshape. The thermal treatment may be performed under an air atmosphere oran oxygen atmosphere so that the second cathode active material particlemay be formed as a lithium metal oxide.

Within the above temperature range, generation of secondary particlesmay be substantially suppressed, and the second cathode active materialparticle without defects therein may be achieved. Preferably, thethermal treatment may be performed at a temperature from about 800° C.to about 1,000° C.

The second cathode active material may be mixed and stirred togetherwith a binder, a conductive material and/or a dispersing agent in asolvent to form a second slurry. The second slurry may be coated on thefirst cathode active material layer 112, and dried and pressed to obtainthe second cathode active material layer 114. The binder and theconductive material substantially the same as or similar to those usedin the first cathode active material layer 112 may also be used.

As described above, the first cathode active material particles and thesecond cathode active material particles having different compositionsor molar ratios may be included in different layers, so that desiredproperties according to layer positions may be effectively implemented.

In exemplary embodiments, the first cathode active material layer 112contacting the cathode current collector 105 may include the lithiummetal oxide having a higher nickel amount than that of the secondcathode active material particle in the second cathode active materiallayer 114. Thus, high capacity/power may be effectively achieved from acurrent through the cathode current collector 105.

The second cathode active material layer 114 that may be exposed at anouter surface of the cathode 100 may include the second cathode activematerial particle having a relatively reduced nickel amount so thatthermal stability and life-span stability may be enhanced.

As described above, the second cathode active material layer 114 mayinclude the second cathode active material particles, each of which mayhave the single particle shape, so that a crack propagation caused whenan external object penetrates the battery may be suppressed to block arapid propagation of thermal energy. Thus, the second cathode activematerial layer 114 may serve as a cathode coating layer providingpenetration stability.

The first cathode active material layer 112 may include the firstcathode active material particles having the concentrationnon-uniformity region or the concentration gradient, and thus thermalstability and life-span stability at the surface of each particle mayalso be improved in the first cathode active material layer 112.Additionally, the first cathode active material particles may have asecondary particle structure in which, e.g., rod-type individual primaryparticles are aggregated, so that an ion mobility between the individualprimary particles may be promoted, and charging/discharging rate andcapacity retention may be improved.

In some embodiments, a diameter (e.g., D50 of a cumulative volumetricparticle size distribution) of the second cathode active materialparticles may be smaller than a diameter of the first cathode activematerial particles. Accordingly, packing property in the second cathodeactive material layer 114 may be enhanced, and propagation of heat andcracks due to penetration or pressing may be more effectively suppressedor reduced.

For example, an average particle diameter of the second cathode activematerial particles may be about 1 to 12 μm, preferably about 1 to 10 μm,and more preferably about 2 to 8 μm. An average particle diameter of thefirst cathode active material particles may be about 8 to 30 μm.

In some embodiments, the first cathode active material particle and/orthe second cathode active material particle may further include a dopingor a coating on the surface thereof. For example, the doping or thecoating may include Al, Ti, Ba, Zr, Si, B, Mg, P, W, Na, V, Cu, Zn, Ge,Ag, Ba, Nb, Ga, Cr, Sr, Y, Mo, an alloy thereof or on oxide thereof.These may be used alone or in a combination thereof. The first cathodeactive material particle and/or the second cathode active materialparticle may be passivated by the doping or the coating so thatpenetration stability and life-span of the battery may be furtherimproved.

In some embodiments, a thickness of the second cathode active materiallayer 114 may be less than that of the first cathode active materiallayer 112. Accordingly, the second cathode active material layer 114 mayserve as a coating layer providing a penetration barrier, and the firstcathode active material layer 112 may serve as an active layer providingpower/capacity.

For example, the thickness of the first cathode active material layer112 may be in a range from about 50 μm to about 200 μm. The thickness ofthe second cathode active material layer 114 may be in a range fromabout 10 μm to about 100 μm.

In exemplary embodiments, an adhesive force of the first cathode activematerial layer 112 to the surface of the cathode current collector 105may be greater than an adhesive force of the second cathode activematerial layer 114 to the surface of the first cathode active materiallayer 112.

Thus, the first cathode active material particles having a relativelyhigh content of Ni and having a secondary particle structure may be morestably attached to the cathode current collector 112. Accordingly,generation of gas during repeated high-temperature charging anddischarging may be suppressed.

In exemplary embodiments, a SPAN value in the second cathode activematerial layer 114 or a SPAN value of the second cathode active materialparticles may be greater than a SPAN value in the first cathode activematerial layer 112 or a SPAN value of the first cathode active materialparticles.

The term “SPAN” used herein may refer to a ratio of a difference betweena D90 particle diameter and a D10 particle diameter relative to a D50particle diameter as expressed by Equation 1 below.

$\begin{matrix}{{SPAN} = {\left( {{D90} - {D10}} \right)/D50}} & \left\lbrack {{Equation}1} \right\rbrack\end{matrix}$

In Equation 1, D10, D50 and D90 are particle diameters at cumulativevolume percents of 10%, 50% and 90%, respectively, in a cumulativevolume particle diameter distribution of active material particles inwhich various particle diameters are distributed.

In exemplary embodiments, a ratio of a span value of the second cathodeactive material particles relative to a span value of the first cathodeactive material particles may be in a range from 2 to 4.5. In apreferable embodiment, the ratio of the span values may be in a rangefrom 2 to 3.5.

In the range of the above-described span value ratio, damages or cracksin a pressing process performed during the fabrication of the electrodeincluding the cathode active material particles may be effectivelyprevented. Accordingly, generation of gas from the cathode may besuppressed even though charging and discharging are repeatedly performedat high temperature, and operational stability at high temperature andlife-span properties of the secondary battery may be improved.

In some embodiments, the span value of the second cathode activematerial particles may be in a range from 0.7 to 2, preferably from 0.9to 1.7. The span value of the first cathode active material particlesmay be less than 1, preferably from 0.3 to 0.6.

In the range of the above-described span value, a porosity and aspecific surface area of the cathode may be properly maintained toobtain a balance between an ion conductivity and an electrode density.

For example, the span value of the cathode active material particles maybe controlled or changed by particles sizes and element content ratiosof precursors (e.g., an NCM precursor), a temperature, a time or atemperature increasing rate of a heat treatment or an annealing process,etc., employed during the preparation of the active material particles.

FIGS. 2 and 3 are a top planar view and a cross-sectional view,respectively, schematically illustrating a lithium secondary battery inaccordance with exemplary embodiments. Specifically, FIG. 3 is across-sectional view taken along a line I-I′ of FIG. 2 in a thicknessdirection of the lithium secondary battery.

Referring to FIGS. 2 and 3, a lithium secondary battery 200 may includean electrode assembly 150 housed in a case 160. The electrode assembly150 may include the cathode 100, an anode 130 and a separation layer 140repeatedly stacked as illustrated in FIG. 3.

The cathode 100 may include the cathode active material layer 110 coatedon the cathode current collector 105. Although not illustrated in detailin FIG. 3, the cathode active material layer 110 may include amulti-layered structure including the first cathode active materiallayer 112 and the second cathode active material layer 114 as describedwith reference to FIG. 1.

The anode 130 may include an anode current collector 125 and an anodeactive material layer 120 formed by coating an anode active material onthe anode current collector 125.

The anode active material may include a material that may be capable ofadsorbing and ejecting lithium ions. For example, a carbon-basedmaterial such as a crystalline carbon, an amorphous carbon, a carboncomplex or a carbon fiber, a lithium alloy, a silicon-based material,tin, etc., may be used.

The amorphous carbon may include a hard carbon, cokes, a mesocarbonmicrobead (MCMB), a mesophase pitch-based carbon fiber (MPCF), etc.

The crystalline carbon may include a graphite-based material such asnatural graphite, artificial graphite, graphitized cokes, graphitizedMCMB, graphitized MPCF, etc. The lithium alloy may further includealuminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium,or indium.

The anode current collector 125 may include, e.g., gold, stainlesssteel, nickel, aluminum, titanium, copper or an alloy thereof, andpreferably may include copper or a copper alloy.

In some embodiments, a slurry may be prepared by mixing and stirring theanode active material with a binder, a conductive material and/or adispersing agent in a solvent. The slurry may be coated on at least onesurface of the anode current collector 125, dried and pressed to obtainthe anode 130.

Materials substantially the same as or similar to those used in thecathode active material layer 110 may be used as the binder and theconductive material. In some embodiments, the binder for the anode mayinclude, e.g., an aqueous binder such as styrene-butadiene rubber (SBR)for compatibility with the carbon-based active material, and may be usedwith a thickener such as carboxymethyl cellulose (CMC).

The separation layer 140 may be interposed between the cathode 100 andthe anode 130. The separation layer 140 may include a porous polymerfilm prepared from, e.g., a polyolefin-based polymer such as an ethylenehomopolymer, a propylene homopolymer, an ethylene/butene copolymer, anethylene/hexene copolymer, an ethylene/methacrylate copolymer, or thelike. The separation layer 140 may be also formed from a non-wovenfabric including a glass fiber with a high melting point, a polyethyleneterephthalate fiber, or the like.

In some embodiments, an area and/or a volume of the anode 130 (e.g., acontact area with the separation layer 140) may be greater than that ofthe cathode 100. Thus, lithium ions generated from the cathode 100 maybe easily transferred to the anode 130 without loss by, e.g.,precipitation or sedimentation. Therefore, the enhancement of power andstability by the combination of the first and second cathode activematerial layers 112 and 114 may be effectively implemented.

In exemplary embodiments, an electrode cell may be defined by thecathode 100, the anode 130 and the separation layer 140, and a pluralityof the electrode cells may be stacked to form the electrode assembly 150having, e.g., a jelly roll shape. For example, the electrode assembly150 may be formed by winding, laminating or folding of the separationlayer 140.

The electrode assembly 150 may be accommodated in a case 160 togetherwith an electrolyte to form the lithium secondary battery. In exemplaryembodiments, the electrolyte may include a non-aqueous electrolytesolution.

The non-aqueous electrolyte solution may include a lithium salt and anorganic solvent. The lithium salt may be represented by Li⁺X⁻, and ananion of the lithium salt X⁻ may include, e.g., F⁻, Cl⁻, Br⁻, I⁻, NO₃ ⁻,N(CN)₂ ⁻, BF₄ ⁻, ClO₄ ⁻, PF₆ ⁻, (CF₃)₂PF₄ ⁻, (CF₃)₃PF₃ ⁻, (CF₃)₄PF₂ ⁻,(CF₃)₅PF⁻, (CF₃)₆P⁻, CF₃SO₃ ⁻, CF₃CF₂SO₃ ⁻, (CF₃SO₂)₂N⁻, (FSO₂)₂N⁻,CF₃CF₂(CF₃)₂CO⁻, (CF₃SO₂)₂CH⁻, (SF₅)₃C⁻, (CF₃SO₂)₃C⁻, CF₃(CF₂)₇SO₃ ⁻,CF₃CO₂ ⁻, CH₃CO₂ ⁻, SCN⁻, (CF₃CF₂SO₂)₂N⁻, etc.

The organic solvent may include propylene carbonate (PC), ethylenecarbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC),ethylmethyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate,dimethyl sulfoxide, acetonitrile, dimethoxy ethane, diethoxy ethane,vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite,tetrahydrofuran, etc. These may be used alone or in a combinationthereof.

As illustrated in FIG. 2, an electrode tab (a cathode tab and an anodetab) may be formed from each of the cathode current collector 105 andthe anode current collector 125 to extend to one end of the case 160.The electrode tabs may be welded together with the one end of the case160 to be connected to an electrode lead (a cathode lead 107 and ananode lead 127) exposed at an outside of the case 160.

FIG. 2 illustrates that the cathode lead 107 and the anode lead 127protrude from an upper side of the case 160 in a planar view. However,positions of the electrode leads are not specifically limited. Forexample, the electrode leads may protrude from at least one of lateralsides of the case 160, or may protrude from a lower side of the case160. Further, the cathode lead 107 and the anode lead 127 may protrudefrom different sides of the case 160.

The lithium secondary battery may be fabricated into a cylindrical shapeusing a can, a prismatic shape, a pouch shape, a coin shape, etc.

The Lithium secondary battery according to exemplary embodiments mayinclude the cathode of the above-described structure to provide stableoperational reliability at high temperature. Accordingly, the lithiumsecondary battery with high power and high capacity may be implementedutilizing a relatively high charging voltage. For example, stableoperational reliability may be provided at a charging voltage of 4.2 Vor more, preferably in a range from 4.2 V to 4.45 V.

Hereinafter, preferred embodiments are proposed to more concretelydescribe the present invention. However, the following examples are onlygiven for illustrating the present invention and those skilled in therelated art will obviously understand that various alterations andmodifications are possible within the scope and spirit of the presentinvention. Such alterations and modifications are duly included in theappended claims.

Example 1

(1) Preparation of First Cathode Active Material Particle

Precipitates were formed by continuously changing a mixing ratio of anickel precursor and a manganese precursor such that a total averagecomposition was LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂, compositions of a centralportion and a surface portion were LiNi_(0.802)Co_(0.11)Mn_(0.088)O₂ andLiNi_(0.77)Co_(0.11)Mn_(0.12)O₂, respectively, and a concentrationgradient region (decreasing Ni concentration, increasing Mnconcentration) was formed between the central portion and the surfaceportion. After a heat treatment, the first cathode active materialparticles having a secondary particle structure (average diameter (D50):13 μm) (hereinafter, that may be referred to as a particle D) wereprepared.

FIGS. 4 and 5 are SEM images showing a cross-section and a surface of afirst cathode active material particle, respectively, prepared accordingto Example 1.

Referring to FIGS. 4 and 5, the first cathode active material particlehad a structure where numerous primary particles were agglomerated toform a single secondary particle.

(2) Preparation of Second Cathode Active Material Particle

Ni_(0.75)Co_(0.10)Mn_(0.15)(OH)₂ as an NCM precursor and Li₂CO₃ and LiOHas a lithium source were mixed with grinding for about 20 minutes. Themixed powder was annealed at a temperature from 700° C. to 1000° C. for15 hours, and then pulverizing, sieving and de-iron processes wereperformed to obtain a single particle structure (average diameter (D50):5.7 μm) of LiNi_(0.75)Co_(0.10)Mn_(0.15)O₂ (hereinafter, that may bereferred to as a particle A).

FIGS. 6 and 7 are SEM images showing a cross-section and a surface of asecond cathode active material particle, respectively, preparedaccording to Example 1.

Referring to FIGS. 6 and 7, the second cathode active material particlewas formed as a single particle shape, an inside of which was asubstantially single body.

(3) Fabrication of Secondary Battery

A first cathode mixture was prepared by mixing the first cathode activematerial particles, Denka Black as a conductive material and PVDF as abinder in a mass ratio of 94:3:3. The first mixture was coated on analuminum current collector, and then dried and pressed to form a firstcathode active material layer.

A second cathode mixture was prepared by mixing the second cathodeelectrode active material particles, Denka Black as a conductivematerial and PVDF as a binder in a mass ratio of 95.5:3:1.5. The secondcathode mixture was coated on a surface of the first cathode activematerial layer, and then dried and pressed to form a second cathodeactive material layer. Accordingly, a cathode in which the first cathodeactive material layer and the second cathode active material layer weresequentially stacked on the cathode current collector was obtained.

An electrode density of the first cathode active material layer was 3.7g/cc, and a thickness of the first cathode active material layer was 57μm. An electrode density of the second cathode active material layer was3.7 g/cc, and a thickness of the second cathode active material layerwas 30 μm.

An anode slurry was prepared by mixing 93 wt % of natural graphite as ananode active material, 5 wt % of a flake type conductive agent KS6, 1 wt% of SBR as a binder and 1 wt % of CMC as a thickener. The anode slurrywas coated, dried and pressed on a copper substrate to form an anode.

The cathode and the anode obtained as described above were notched witha proper size and stacked, and a separator (polyethylene, thickness: 25μm) was interposed between the cathode and the anode to form anelectrode cell. Each tab portion of the cathode and the anode waswelded. The welded cathode/separator/anode assembly was inserted in apouch, and three sides of the pouch (e.g., except for an electrolyteinjection side) were sealed. The tab portions were also included insealed portions. An electrolyte was injected through the electrolyteinjection side, and then the electrolyte injection side was also sealed.Subsequently, the above structure was impregnated for more than 12hours.

The electrolyte was prepared by preparing 1M LiPF₆ solution in a mixedsolvent of EC/EMC/DEC (25/45/30; volume ratio), and then adding 1 wt %of vinylene carbonate, 0.5 wt % of 1,3-propensultone (PRS), and 0.5 wt %of lithium bis(oxalato) borate (LiBOB).

The lithium secondary battery as fabricated above was pre-charged byapplying a current (5 A) corresponding to 0.25 C for 36 minutes. After 1hour, the battery was degassed, aged for more than 24 hours, and then aformation charging-discharging (charging condition of CC-CV 0.2 C 4.2 V0.05 C CUT-OFF, discharging condition CC 0.2 C 2.5 V CUT-OFF) and astandard charging-discharging (charging condition of CC-CV 0.5 C 4.2V0.05 C CUT-OFF, discharging condition CC 0.5 C 2.5V CUT-OFF) wereperformed.

Examples 2-6 and Comparative Examples

Lithium secondary batteries were fabricated by the same method as thatin Example 1 except that cathode active materials were changed inExamples 2-6 and Comparative Examples as shown in Table 1 below.

Compositions and particle diameters of the cathode active materials areas follows:

i) particle B (single particle): LiNi_(0.65)Co_(0.15)Mn_(0.20)O₂, D50:3.4 μm

ii) particle C (single particle): LiNi_(0.83)Co_(0.12)Mn_(0.05)O₂, D50:7.2 μm

iii) particle E (secondary particle): LiNi_(0.88)Co_(0.1)Mn_(0.02)O₂without a concentration gradient, D50: 15.8 μm

iv) particle F (secondary particle): LiNi_(0.83)Co_(0.12)Mn_(0.05)O₂,without a concentration gradient, D50: 4.1 μm

v) particle H (single particle): LiNi_(0.83)Co_(0.12)Mn_(0.05)O₂, D50:2.8 μm

vi) particle I (single particle): LiNi_(0.83)Co_(0.12)Mn_(0.05)O₂, D50:3.0 μm

vii) particle G (secondary particle): LiNi_(0.88)Co_(0.1)Mn_(0.02)O₂without a concentration gradient, D50: 17.2 μm

TABLE 1 SPAN ratio particle shape SPAN (2nd layer/1st layer) Example 12nd layer (particle A) single particle 0.91 2.01 1st layer (particle D)secondary particle 0.45 Example 2 2nd layer (particle A) single particle0.91 2.44 1st layer (particle E) secondary particle 0.37 Example 3 2ndlayer (particle B) single particle 1.62 3.56 1st layer (particle D)secondary particle 0.45 Example 4 2nd layer (particle B) single particle1.62 4.33 1st layer (particle E) secondary particle 0.37 Example 5 2ndlayer (particle C) single particle 1.17 2.57 1st layer (particle D)secondary particle 0.45 Example 6 2nd layer (particle C) single particle1.17 3.12 1st layer (particle E) secondary particle 0.37 Comparative 2ndlayer (particle H) single particle 2.11 4.64 Example 1 1st layer(particle D) secondary particle 0.45 Comparative 2nd layer (particle H)single particle 2.11 5.64 Example 2 1st layer (particle E) secondaryparticle 0.37 Comparative 2nd layer (particle I) single particle 0.601.32 Example 3 1st layer (particle D) secondary particle 0.45Comparative 2nd layer (particle I) single particle 0.60 1.61 Example 41st layer (particle E) secondary particle 0.37 Comparative 2nd layer(particle I) single particle 0.60 1.82 Example 5 1st layer (particle G)secondary particle 0.33 Comparative 2nd layer (particle F) secondaryparticle 2.22 4.89 Example 6 1st layer (particle D) secondary particle0.45 Comparative 2nd layer (particle F) secondary particle 2.22 5.94Example 7 1st layer (particle E) secondary particle 0.37 Comparative 2ndlayer (particle G) secondary particle 0.33 0.72 Example 8 1st layer(particle D) secondary particle 0.45 Comparative 2nd layer (particle G)secondary particle 0.33 0.87 Example 9 1st layer (particle E) secondaryparticle 0.37 Comparative single-layered structure of particle D — —Example 10

Experimental Example

(1) Measurement of SPAN Value

Images of the first layer (lower layer) and the second layer (upperlayer) were obtained using an SEM (Scanning Electron Microscopy) afteran ion milling of the cathodes prepared according to Examples andComparative Examples. In a magnification of ×1,000 from the obtainedimages, 30 particles included in an area of 25 μm (width)×25 μm (length)were selected from the second layer, and all particles from which alength could be substantially measured were selected from the firstlayer. Lengths of the selected particles were measured using an SEMlength measurement device (FE-SEM (FEI Apreo), manufactured by Hitachi).

SPAN values of the first layer and the second layer were calculatedusing D10, D50 and D90 measured from each of the first layer and thesecond layer.

(2) Evaluation on Gas Generation

Each secondary battery of Examples and Comparative Examples was chargedwith SOC100% (CC-CV 1.0 C 4.2V 0.05 C CUT-OFF), and an amount of gasgenerated at an inside the battery cell was measured after storage at a60° C. chamber for predetermined week intervals.

(3) Evaluation on Life-Span (Capacity Retention) Property

Charging with SOC100% (CC-CV 1.0 C 4.2V 0.05 C CUT-OFF) and discharging(CC 1.0 C 2.5V CUT-OFF) of the lithium secondary batteries of Examplesand Comparative Examples were repeated 500 times in a chamber at 45° C.,and then a capacity retention was measured by a percentage (%) of adischarge capacity at 500th cycle relative to a discharge capacity at1st cycle.

The results are shown in Table 2 below.

TABLE 2 Capacity Gas generation after 60° C. storage Retention 0 week 8weeks 12 weeks 16 weeks (45° C.) Example 1 0 27 34 54 91% Example 2 0 2533 51 93% Example 3 0 33 40 60 90% Example 4 0 30 38 55 93% Example 5 020 31 47 93% Example 6 0 19 29 43 94% Comparative 0 51 76 108 82%Example 1 Comparative 0 44 64 83 87% Example 2 Comparative 0 48 71 10583% Example 3 Comparative 0 45 65 92 87% Example 4 Comparative 0 45 6791 87% Example 5 Comparative 0 74 104 142 71% Example 6 Comparative 0 7099 135 75% Example 7 Comparative 0 68 101 129 77% Example 8 Comparative0 62 93 121 80% Example 9 Comparative 0 92 127 145 77% Example 10

Referring to Table 2, in Examples having the multi-layered structure ofsecondary particles-single particles and having the predetermined rangesof the SPAN ratios, improved storage stability and life-span stabilitywere provided.

What is claimed is:
 1. A cathode for a lithium secondary battery,comprising: a cathode current collector; and a cathode active materiallayer comprising a first cathode active material layer and a secondcathode active material layer sequentially stacked on the cathodecurrent collector, wherein the first cathode active material layercomprises first cathode active material particles having a secondaryparticle structure, and the second cathode active material layercomprises second cathode active material particles having a singleparticle shape, and wherein a ratio of a SPAN value defined as Equation1 of the second cathode active material particles relative to a SPANvalue defined as Equation 1 of the first cathode active materialparticles is in a range from 2 to 4.5: $\begin{matrix}{{SPAN} = {\left( {{D90} - {D10}} \right)/D50}} & \left\lbrack {{Equation}1} \right\rbrack\end{matrix}$ wherein, in Equation 1, D10, D50 and D90 are particlediameters at cumulative volume percents of 10%, 50% and 90%,respectively, in a cumulative volume particle diameter distribution. 2.The cathode for a lithium secondary battery of claim 1, wherein theratio of the SPAN value of the second cathode active material particlesrelative to the SPAN value of the first cathode active materialparticles is in a range from 2 to 3.5
 3. The cathode for a lithiumsecondary battery of claim 1, wherein the SPAN value of the secondcathode active material particles is in a range from 0.7 to 2, and theSPAN value of the first cathode active material particles is lessthan
 1. 4. The cathode for a lithium secondary battery of claim 3,wherein the SPAN value of the second cathode active material particlesis in a range from 0.9 to 1.7, and the SPAN value of the first cathodeactive material particles is in a range from 0.3 to 0.6.
 5. The cathodefor a lithium secondary battery of claim 1, wherein each of the firstcathode active material particles and the second cathode active materialparticles comprises a lithium metal oxide containing nickel, and whereina content of nickel comprised in the first cathode active materialparticles is greater than or equal to a content of nickel comprised inthe second cathode active material particles.
 6. The cathode for alithium secondary battery of claim 1, wherein each of the first cathodeactive material particles and the second cathode active materialparticles further contains cobalt and manganese, wherein a molar ratioof nickel among nickel, cobalt and manganese in the first cathode activematerial particles is 0.6 or more, and wherein a molar ratio of nickelamong nickel, cobalt and manganese in the second cathode active materialparticles is 0.5 or more.
 7. The cathode for a lithium secondary batteryof claim 6, wherein the molar ratio of nickel among nickel, cobalt andmanganese in the first cathode active material particles is 0.8 or more,and wherein the molar ratio of nickel among nickel, cobalt and manganesein the second cathode active material particles is 0.6 or more.
 8. Thecathode for a lithium secondary battery of claim 6, wherein the firstcathode active material particles comprise a concentrationnon-uniformity region of at least one element of nickel, cobalt andmanganese between a central portion and a surface portion.
 9. Thecathode for a lithium secondary battery of claim 6, wherein each ofnickel, cobalt and manganese contained in the second cathode activematerial particles does not form a concentration gradient.
 10. Thecathode for a lithium secondary battery of claim 1, wherein an averageparticle diameter of the second cathode active material particles issmaller than an average particle diameter of the first cathode activematerial particles.
 11. The cathode for a lithium secondary battery ofclaim 1, wherein a thickness of the second cathode active material layeris smaller than a thickness of the first cathode active material layer.12. A lithium secondary battery, comprising: a case; and an electrodeassembly accommodated in the case, the electrode assembly comprising ananode and the cathode for a lithium secondary battery according to claim1 facing the anode.